Efficient irrigation is required for high crop productivity, control of soilborne pathogens, and environmental water quality. In the San Joaquin and Central Coast production regions, typically 1.5 to 2.5 feet of water is applied annually, with 4 to 5 feet applied in the Imperial Valley. In some areas of the Delta, asparagus crops may be supplied with much of their water from a shallow water table.
A good irrigation program follows these basic guidelines.
- Monitor soil moisture using the feel and appearance method or tools such as tensiometers or gypsum blocks.
- Apply irrigation water according to crop evapotranspiration (ETc) to assure plant needs are met through all growth stages and reduce incidence of diseases such as crown and spear rot.
- Keep records of irrigation and rainfall amounts.
- Do not irrigate deeper or more frequently than necessary.
- Avoid applying too much water to prevent soil oversaturation (soil with moisture in excess of levels recommended for the current stage of growth).
- Use efficient irrigation systems designed to give uniform water distribution.
Irrigation frequency varies with the climate, soil type and water table, but ranges from 10 to 30 days or more. Typically 3 to 6 inches of water is applied at each irrigation, depending on soil texture and the degree of water depletion since the previous irrigation.
Subsurface drip irrigation systems are the most efficient, typically requiring 10 to 20% less water than sprinklers, and they can be used to apply fertilizer and some pesticides (e.g., fertigation or chemigation). They also allow for irrigation during the harvest period without interfering with crews and equipment. Furthermore, this irrigation method eliminates surface runoff. However, drip systems require significant resources to maintain, and management may be particularly challenging when used on very coarse or very heavy soils. Rodents may need to be controlled to prevent damage to buried drip systems.
One line of drip tape per bed is placed 3 to 6 inches below the depth of the crown. Use tapes at least 10 ml thick to prevent puncture; thicker is better. Time between drip irrigations can range from once per week early in the season to 2 to 3 times per week during peak water demand.
IRRIGATION AT THE DIFFERENT GROWTH STAGES
During the first year, the crop is small, so evapotranspiration and the crop coefficient are low; therefore, irrigation is required infrequently.
Spear growth to harvest
In desert regions where rainfall is low, irrigations are scheduled preharvest to moisten soil and periodically during harvest. Irrigating during harvest is also recommended for crops in Central Coast areas due to the long harvest period. In furrow-irrigated fields, irrigation is often applied to alternate furrows so workers can walk through dry furrows instead of bed tops.
During the fern stage, irrigate to match the ETc demand of the crop. See below for information on scheduling irrigation. As the fern begins to senesce in the fall, cease irrigation to deter new growth as the crop approaches dormancy.
Depending on the growing region, the soil profile is replenished during dormancy by winter rains, irrigation, or flooding.
Timing of irrigation during the growing season is based on soil moisture measurements and a water budget, which can be determined using evapotranspiration data. A combination of methods is usually best and disease potential must be considered.
Monitoring soil moisture
A major factor in determining the allowable soil moisture depletions is soil texture; clay loam soils hold more water than do sandy loams and, therefore, have longer intervals between irrigations. For typical asparagus plantings, avoid exceeding depletion of more than 50% of the soil's water holding capacity; that is equivalent to approximately 40 to 80 centibars of tension. Detailed information on determining allowable soil moisture depletions and on developing an irrigation schedule is in Scheduling Irrigations: When and How Much Water to Apply.
Always check soil moisture before applying water and estimate how much available water remains in the crop rooting depth. Use a soil tube to take soil from the rooting zone at several points in each field. The feel and appearance of the soil as outlined in the table below can be used as a guide for judging the depletion level in soil taken from the root zone.
|Coarse-textured soils||Inches of water needed1||Medium-textured soils||Inches of water needed1||Fine-textured soils||Inches of water needed1|
|Soil looks and feels moist, forms a cast or ball, and stains hand.||0.0||Soil dark, feels smooth, and ribbons out between fingers; leaves wet outline on hand.||0.0||Soil dark, may feel sticky, stains hand; ribbons easily when squeezed and forms a good ball.||0.0|
|Soil dark, stains hand slightly; forms a weak ball when squeezed.||0.3||Soil dark, feels slick, stains hand; works easily and forms ball or cast.||0.5||Soil dark, feels slick, stains hand; ribbons easily and forms a good ball.||0.7|
|Soil forms a fragile cast when squeezed.||0.6||Soil crumbly but may form a weak cast when squeezed.||1.0||Soil crumbly but pliable; forms cast or ball, will ribbon; stains hand slightly.||1.4|
|Soil dry, loose, crumbly.||1.0||Soil crumbly, powdery; barely keeps shape when squeezed.||1.5||Soil hard, firm, cracked; too stiff to work or ribbon.||2.0|
|1||Amount needed to restore 1 foot of soil depth to field capacity when soil is in the condition indicated.|
Water needs may vary from one part of a field to another, especially if the field includes different soil types or slopes. Plants in a sandy streak or where water holding capacity is less, may show stress sooner than the rest of the crop. Watch these weak areas to gain advance notice of when irrigation is needed for the rest of the field. However, schedule irrigations according to the need shown by most of the crop.
Instruments are available that measure the moisture content of the soil such as tensiometers, gypsum blocks, and neutron probes. To obtain reliable readings, you must install these instruments in areas representative of the field, or spots where water stress occurs more readily. At each site, install one soil moisture probe at the rooting depth of the current growth stage, and a second probe 24 inches deep. Follow the recommendations of irrigation experts in using soil probes for irrigation scheduling. Aerial infrared photography can also help identify areas of moisture stress within fields.
Soil moisture can be monitored indirectly using a water budget to estimate how much water the crop uses daily, considering climatic conditions, planting time, and crop season. However, while water budgets provide estimates of crop water use, there are many variable factors such as cultivar, disease, weeds, insects, physical characteristics of individual fields, and management factors that affect actual water use. Using a direct measurement of soil moisture is recommended when making the final decision about when to irrigate.
Crop evapotranspiration (ETc) is the crop water use, determined by evaporation of water from the soil and transpiration of water from plant leaves. If you know how much available water is in the crop rooting depth at field capacity and how much is lost through ET each day, you can estimate the remaining water at any time by adding the daily ET values.
Calculate the ETc between irrigations with the equation: ETc = Kc × ETo where Kc is the crop coefficient, and ETo is the reference crop evapotranspiration, which can be found on the CIMIS website (CIMIS = California Irrigation Management Information System) in daily, real-time, or monthly average values.
Soil salinity is the level of soluble salts present; low salinity is desirable. As soil salinity increases, the likelihood of crop damage increases and remedial actions are justified. Actions include leaching the soil profile, using overhead sprinklers to create a zone of lower concentration around the seedlings or transplants, and switching to a higher-quality irrigation source to prevent further buildup (high soil salinity could be the result of using marginal-quality irrigation water). However, studies have shown asparagus is quite salt-tolerant, with soil salinity values of up to 6 dS/m (deciSiemens per meter) having little to no impact on yield.